Platypus genome reveals much about sex

The platypus, with its webbed feeet and duck' bill, has always been a source of wonder in the mammal world. But now its DNA is helping to unravel some mysteries of human evolution (Source: Ian Elton/Healesville Sanctuary)

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Genome sequencing of Australia's platypus, an animal so unusual it was thought to be a hoax when sent to Europe in the 18th century, has moved the evolution of sex determination in humans forward more than 160 million years.

The research, which involved more than 100 researchers worldwide, could also lead to development of new antibiotics and therapeutics, says Dr Kathy Belov of Australia's University of Sydney.

Bevlov is one of more than 20 Australians involved in the research, which is released today in a series of papers in the journals Nature and Genome Research.

Co-author of the Nature paper Professor Jenny Graves, from the Australian National University's (ANU) Comparative Genomics Research Group, says understanding the genome of the duck-billed platypus helps fill in the gaps of how mammals evolved.

The platypus is unique because it has a duck's bill, webbed feet, lives in water and lays eggs.

Yet because it has fur and produces milk for its young it is considered a mammal.

"The platypus is no more ancient than we are," Graves says.

"But the common ancestor was the intermediary between reptiles and mammals and the platypus gives us clues as to what our ancient ancestor looked like."

She says by comparing the platypus genome with the DNA of humans, kangaroos, mice and chickens, researchers can work out when genes and traits specific to mammals emerged.

"It's like being able to dig down in time," says Graves, who says she is 'floored' by the sex determination finding.

"The platypus is telling us that all ancient mammals [determined sex] like birds," she says.

"The chromosomes that became our X and Y chromosomes were there but they didn't have anything to do with sex."

What determines an animal's sex?

ANU colleague Dr Paul Waters says before the sequencing it was believed that sex determination in mammals stretched back to when birds and mammals diverged about 350 million years ago.

But he writes in Genome Research that sex chromosome mapping of the platypus DNA shows that the platypus lacks the sex-determining gene SRY found in all mammals.

So he says the platypus sex chromosomes are more similar to those of birds then mammals.

This means, he says, the human sex chromosomes must have evolved after the platypus diverged from the mammalian lineage about 166 million years ago.